The present invention relates generally to microwave couplers which convert one input signal into two output signals. Specifically, the present invention relates to a coupler which produces two isolated output signals in-phase with each other at typically unequal power levels. Furthermore, the present invention permits power division at a ratio of at least 13 dB between the outputs while using common manufacturing tolerances and requiring only a small amount of space.
Various image element and other multi-element antenna systems known in the art may advantageously use the present invention. These antenna systems incorporate a multiplicity of antenna elements arranged in a specific configuration to achieve a desired radiation pattern. One feed signal typically energizes the entire antenna system through a feed network which divides the feed signal into a multiplicity of sub-signals at various phase and power level relationships to one another. Each of these sub-signals in turn energizes an antenna element.
The typical feed network divides the one feed signal into the multiplicity of sub-signals through the use of a binary division scheme. Thus, a typical feed network uses a multiplicity of couplers each of which convert an input signal into two output signals. Accordingly, such a coupler may divide a feed signal into first and second intermediate signals. Another such coupler may then divide the first intermediate signal into third and fourth intermediate signals. Likewise, another such coupler may divide the second intermediate signal into fifth and sixth intermediate signals. This process of dividing one signal into two signals continues until a sub-signal for each element in the antenna system is provided.
Design requirements resulting from miniaturization and complication of image element and other multi-element antenna systems necessitate the use of high performance couplers. As an antenna system incorporates more elements in a smaller area the cross-coupling between the elements increases. Thus, couplers used in the feed network must satisfactorily isolate one element from another. Additionally, the power division ratio which a coupler must produce between the coupler's output signals increases as a result of such design requirements. For example, the couplers may be required to provide a 13 dB power ratio between the coupler's outputs. Furthermore, complicated antenna systems require that the couplers accurately deliver the designed power and phase relationships. Thus, the inevitable variations in a coupler's dimensions that occur within reasonable manufacturing tolerances must not substantially affect such relationships.
Although the prior art teaches couplers that could be adapted to many of the high performance requirements mentioned above, such teaching fails fo suggest an entirely satisfactory coupler. Thus, the Wilkinson Power Divider, the Magic TEE (or Unequal Hybrid Ring), the Coupled Line Directional Couplers, and the Uneven Power Split Coupler described in the NASA Technical Memorandum No. 81,870, August 1980, each fail to demonstrate satisfactory performance.
The well known Wilkinson Power Divider represents a junction having three ports. A first port serves as an input, while second and third ports represent the ends of coupler legs and operate as outputs. A resistor connects between the coupler legs at the second and third ports. The Wilkinson Power Divider accomplishes an unequal power division by using legs having different impedances and accomplishes isolation between the outputs through the use of the resistor.
However, the Wilkinson Power Divider causes problems at higher ratios of power division. A miniaturized antenna system using stripline or microstrip construction techniques requires reasonably small conductive strips. Conversely, the conductive strips must be large enough so that variations within achievable manufacturing tolerances do not produce a significant effect. Thus, a 50 ohm line represents an advantageous compromise because it typically uses a 0.040 inch wide conductive strip having a manufacturing tolerance of 0.003 inch. The Wilkinson Power Divider problem occurs because at higher ratios of power division one of the coupler legs must have a very high impedance. For example, such a high ratio power division might require a coupler leg only 0.005 inch wide. Since the typical manufacturing tolerance is 0.003 inch, a designer could expect only a 60% accuracy on the power ratio. Using costly high precision manufacturing techniques, the tolerance could be pushed to 0.001 inch. However, such costly techniques would provide only a 20% accuracy on the power ratio which is still inadequate. Thus, the Wilkinson Power Divider becomes ineffective when small width conductive strips are used.
The well known Magic TEE, or Unequal Hybrid Ring using stripline or microstrip techniques, represents a junction having four ports. A first port serves as the coupler input. A quarter-wavelength, high impedance coupler leg connects the first port to a second port, and a quarter-wavelength, low impedance coupler leg connects the first port to a third port. The second and third ports operate as coupler outputs. Another quarter-wavelength, high impedance leg connects the third port to a fourth port, and a three-quarters-wavelength, low impedance leg completes a ring by connecting the fourth port to the second port. The fourth port terminates into a resistor to provide the isolation function. Thus, the ratio of the impedances between the low and high impedance legs determines the power division ratio.
The Magic TEE also fails to meet design needs at high power division ratios. Since low and high impedance legs determine the power ratio, the Magic TEE suffers from similar problems as the Wilkinson Power Divider. Thus, at high ratio power divisions either the low impedance legs are so large that they force the coupler to use too much space, or the high impedance legs are so small that they cannot be accurately maunfactured.
The well known Coupled Line Directional Couplers suggest another power division technique. The field of a first transmission line couples to a second transmission line when the two lines are placed close enough together for a suitable coupling length, which is typically a quarter-wavelength. Here, the gap between the lines determines the power division ratio. Again, the achievable manufacturing tolerances on a gap between two lines prevents these couplers from being effective at power ratios above 8 dB.
The Uneven Power Split Coupler described in NASA Technical Memorandum No. 81,870 suggests a technique for achieving a manufacturable high ratio power division between the outputs of a coupler. However, it also fails to meet design needs because it does not suggest how to isolate the outputs. Thus, such a coupler would degrade performance of a miniaturized antenna system because cross-coupling between the elements would cause unwanted signals to propagate through the feed network. Since a system relies on complex and highly accurate radiation patterns, the propagation of unwanted signals through the feed network would tend to negate the desired effect.